This invention relates to method for measuring a characteristic of a semiconductor memory device, and more particularly to method for a measuring power supply current of a semiconductor memory device more precisely.
In general, various kinds of characteristics are described in a specification of a semiconductor memory device (e.g., RAM, i.e., random access memory). The characteristics include absolute maximum ratings, recommended operating conditions, electrical characteristics, and so on. These characteristics are important to designers who wish to evaluate semiconductor memory devices, to purchasers who are selecting better semiconductor memory devices before purchasing devices, and so on. One of the important characteristics is the power supply current, i.e., the power supply current in operating state and the power supply current in the standby state (standby power supply current). For example, a specification of a static RAM says that its standby power supply current is 10 mA at maximum. A designer who designs a product using a SRAM knows the data (10 mA maximum) from a specification and designs the product based upon the data. The data usually contains some clearance or margin. That is, some margin is added on actually measured standby current. If the measured standby current is wrong and the maximum standby current is higher than the measured one, the margin becomes smaller. In such a case, it may be risky for a designer to design a product based upon data shown in a specification. A similar situation occurs in purchasing semiconductor memory devices. A person, who is in charge of purchasing, generally uses characteristics of the specification for selecting devices which should be purchased. If such characteristics are wrong or are not correct, his purchase decision may be wrong.
Therefore, it is very important to know correct data, or to measure data correctly.
There are two cases to consider in measuring the power supply current to be consumed by a semiconductor memory device. One is a case wherein the measurement is conducted when a semiconductor memory device is operated, i.e., is in chip selection mode. The other is a case wherein the measurement is conducted when a semiconductor memory device is in a standby state, or chip non-selective mode. In these cases, particularly in the measurement in chip non-selection mode, the measurement is conducted regardless of what kind of data each memory cell of the memory device holds.
Generally, it is known that each memory cell tends to hold an arbitrary value of "0" or "1" when the power supply voltage is simply applied to a semiconductor memory device and, data has not yet been written in the memory device. This means that since each memory cell tends to reach a more stabilized state, it holds data of a "0" or "1". Each memory cell is stabilized under the situation that the leakage current from a memory cell to GND (Ground) or to V.sub.DD (power supply voltage) terminal is minimized after each memory cell holds one of "0" and "1".
A conventional measurement of power supply current is conducted under such a stabilized situation. Since the measurement is conducted under a static stabilized state, it is difficult to obtain a more precise maximum standby power supply current. That is, in case that a memory cell holds certain data, it is in more stabilized state than a case wherein it holds opposite data, i.e., data which are opposite to data which are stored in a memory cell under a stabilized state so that the power supply current, particularly standby current which is measured under the stabilized state becomes a lower current value.
In other words, the measured current value differs from the current value of a memory cell in the case that where some data is intentionally written in a memory cell. Therefore, it is difficult, in a conventional measurement method, to know the correct maximum value of the standby power supply current.